1. Field of the Invention
The present invention relates to an optical recording and reproduction apparatus, and more particularly to focusing control and tracking control in an optical pickup device provided in an optical recording and reproduction apparatus.
2. Description of the Background Art
An optical recording and reproduction apparatus includes an optical pickup device as a device for recording information on a disc medium in a stable manner and reproducing the information with fidelity. The optical pickup device is equipped with an objective lens driving mechanism (hereinafter, also referred to as an actuator) for focusing control and tracking control. Here, focusing control refers to causing an objective lens to follow wobbling of a disc surface, the lens collecting a laser beam from semiconductor laser for irradiation of a target position on the disc medium, while tracking control refers to causing the objective lens to follow decentering from a track. The optical pickup device automatically adjusts the objective lens such that a relative positional relation between a target position on the disc medium and a laser beam spot is always maintained constant.
Focusing control and tracking control are realized by detecting return light of the beam collected by the objective lens for irradiation of the disc medium. For example, as disclosed in Japanese Patent Laying-Open Nos. 6-176371 and 2-5228, normally, when a light receiving element such as a photodiode receives return light of the beam, an actuator is operated based on an output from the light receiving element. That is, feedback control is achieved.
FIG. 7 shows a configuration of a main portion of a conventional optical recording and reproduction apparatus disclosed in Japanese Patent Laying-Open No. 6-176371.
Referring to FIG. 7, the optical recording and reproduction apparatus includes an optical head 100 arranged so as to face an optical disc 1, a focus error detection portion 110, a tracking error detection portion 120, analog-digital converters (hereinafter, also referred to as A/D converter) 130, 140, a processor 200, digital-analog converters (hereinafter, also referred to as D/A converter) 80, 81, and drivers 90, 91.
Optical head 100 includes a focus actuator 103, a tracking actuator 104, and an optical system 102.
Focus error detection portion 110 is constituted of a not-shown photodiode portion and an amplifier unit. In the photodiode portion, a photodiode (not shown) divided into four regions receives reflected light from optical disc 1. Then, sum signals (a+c) and (b+d) are obtained by calculating sums of outputs from one pair (a, c) and another pair (b, d) of regions located diagonal to each other among these four regions (denoted as a, b, c, and d, for example).
The amplifier unit receives sum signals (a+c) and (b+d) output from the photodiode portion as inputs, and outputs an output voltage proportional to a difference between the input signals (a+c)−(b+d) as a focus error signal fe.
Tracking error detection portion 120 is also constituted of a not-shown photodiode portion and an amplifier unit. In the photodiode portion, when a photodiode (not shown) divided into two regions (denoted as e, f, for example) receives reflected light from the optical disc, signals e and f are output in accordance with an amount of reflected light.
The amplifier unit receives output signals e, f from the photodiode portion as inputs, and outputs an output voltage proportional to a difference between the input signals (e−f) as a tracking error signal te.
Focus error signal fe and tracking error signal te are converted to digital focus error signal FE and tracking error signal TE in A/D converters 130, 140 respectively, for input to processor 200.
Processor 200 includes a focus control unit 70 generating a focus drive signal FD based on focus error signal FE and a tracking control unit 71 generating a tracking drive signal TD based on tracking error signal TE. Generated focus drive signal FD and tracking drive signal TD are converted to analog signals by D/A converters 80, 81 respectively, for input to drivers 90, 91.
Driver 90 drives focus actuator 103 based on focus drive signal FD so that a beam spot focuses. Driver 91 drives tracking actuator 104 based on tracking drive signal TD so that a beam spot 101 is positioned in the center of a track.
Here, focus error signal fe and tracking error signal te output from error detection portions 110, 120 respectively tend to exhibit variation in gain and offset, depending on variation in optical disc 1 and optical system 102. Accordingly, A/D converters 130, 140 receiving these error signals fe, te need to have a wide dynamic range so as to accept also such variation in processor 200. On the other hand, an A/D converter with a wide dynamic range is expensive, resulting in an increase in the cost of the apparatus.
As shown in FIG. 7, a portion for adjusting gain and offset of error signals fe, te is provided in processor 200. An output from an error detection portion gain/offset adjustment portion 150 can be used to automatically correct variation in the gain and offset in each of error detection portions 110, 120.
With such a configuration, a dynamic range of A/D converters 130, 140 can be narrowed, and reduction in the cost of the apparatus can be achieved.
On the other hand, in the optical recording and reproduction apparatus described above, error signals fe, te input to A/D converters 130, 140 respectively are generated by performing addition and subtraction of the outputs based on the light received at the photodiode portions in focus error detection portion 110 and tracking error detection portion 120 respectively. Therefore, a dynamic range sufficient for taking in a calculation result is necessary in A/D converters 130, 140, which means that the apparatus is still expensive. For example, when output amplitude (peak to peak value) of the output based on the received light is assumed to be 1V, the output amplitude of focus error signal fe is comparable to 4V. Here, the A/D converter needs to have a dynamic range for covering this amplitude.
In connection with large output amplitude of the error signal, a range of adjustment of the offset and gain also becomes wider. Accordingly, control of these factors becomes complicated, resulting in cost increase.